Repost, new thread
Someone may have a solution to this. I'm in the process of isolating my engine so electrically it is above earth. I've had the alternator modified so it is isolated, replaced single wire senders/switches with two wire ones, installed a solenoid for the starter, and ran all earth wires what were using the engine for ground directly to the battery. So except momentarily when the engine is starting there is no electrolysis (steel boat). I used Seabis to find several additional engine-related sources of electrolysis which I can fix. Except one.

I have an external regulator (Balmar ARS-5). Rather than run the ignition wire to the alternator directly through the ignition switch (rightly or wrongly, I was concerned about the amount of current), I used a little auto-type relay. Strangely, the ground for the coil circuit in the relay is going to metal. It is showing electrical continuity with the hull as tested with my multimeter, as well as indicating a critical fault on the Seabis.

The relay case is not grounded. The only thing I can imagine is that it is coming from the positive side of the coil circuit (some kind of reverse current flow?.) Can his be addressed with a diode? Or remove the relay altogether? I hace no idea of the current ratings for the ignition switch or the the ignition wire, but the ignition wire is somewhat heavier than the switch wires.

does that reg have power, ground, and ign? if so the ign will not draw any power. it's just a turn on signal. the power will come from the battery / power wire. through the reg, and out the field wire to the alt.

I'm not sure why you are trying to run ign power to the alt itself. I hope this relay is not on the field wire between alt and reg. the reg will have no control and your alt will either be off or 100% output.